The invention relates to a method and a device for characterizing wafers during the production of solar cells.
As is known, a solar cell encompasses a semiconductor material, in which sunlight is converted into electric current by means of optical absorption. During the production of solar cells, semiconductor wafers are used, from which in each case one solar cell or a plurality of solar cells are produced in a production process. A criterion for the quality of the wafers used for the production of solar cells is, in particular, the lifetime of charge carriers created in the wafer by means of light absorption.
The lifetime is a measure for the period which passes until a created charge carrier returns into its energetic initial state (recombined). In principle, a higher efficiency is thus to be expected when using a wafer with a high charge carrier lifetime for the production of the solar cells.
Typically, silicon is used as the base material for solar cells. The lifetime of charge carriers created by exposure to light is dominated in silicon by means of recombination of the charge carriers via energy states, which are based on foreign atoms (that is, impurities) or on crystal defects. In a wafer comprising few impurities and crystal defects, respectively, generated charge carriers will have a higher lifetime, because fewer recombination centers are available.
A difficulty in assessing the quality of a wafer by means of a lifetime measurement is that the lifetime of charge carriers created in the wafer is influenced by recombination via surface conditions. In particular, in thin wafers, the measurable lifetime is dominated by the surface influence so that the determination of the lifetime would mainly provide a conclusion regarding the surface of the wafer. However, the lifetime of the charge carriers created in the volume of the wafer is crucial for a quality assessment of a wafer by means of the lifetime.
Only an effective total lifetime, which is a function of a share of the surface recombination (with a corresponding recombination time τS) as well as a function of a share of a recombination in the volume of the wafer (with recombination time τbulk), can be determined by means of measurement techniques. The following applies for the effective lifetime τeff:
      I          τ      eff        =            I              τ        S              +          l              τ        bulk            
For the quality determination of a wafer per lifetime measurements, surface effects must thus be reduced and eliminated as far as possible, respectively. In addition to the wafer thickness W, the surface recombination speed S plays a role for the surface recombination time τS. In particular, the estimation applies (if S<D/4W):
      l          τ      S        =            2      ⁢                          ⁢      S        W  
A possibility for reducing surface effects (that is, the surface recombination speed S) lies in providing the wafer surface with a passivation in the form of a dielectric coating (e.g. consisting of oxide, nitride or a polymer). The number of the surface conditions is thus reduced and/or a band deflection is induced at the surface, which reduces the speed of the recombination via surface conditions.
The problem, which is to be solved by the instant invention, is to enable a determination of the wafer quality during the solar cell production by means of lifetime measurement, which is more accurate than was the case previously.